The cloud point of petroleum oil, as defined by the American Society of Testing and Materials (ASTM) standard method D-2500, is the temperature at which haziness caused by formation of small crystals is first observed in a sample of oil which is cooled under prescribed conditions. The method requires that the sample be cooled in a series of constant temperature baths until the cloud point appears. The temperature of each bath, and the temperature at which the sample is transferred from one bath to the next one of lower temperature, are specified in the method.
The cooling rate of an oil sample treated in the manner described above is cyclical. The rate is highest after the sample is transferred due to the large difference in the temperature between the bath and the oil sample. From then on the cooling rate decreases as a function of time until the next transfer takes place. These different cooling rates give rise to inaccuracies of measurement; the true cloud point should be obtained under slow cooling. The preferred cooling rate for petroleum oils is one degree Centigrade per minute, or less. At cooling rates higher than this value the observed cloud point has a tendency to increase with increasing cooling rate and the precision of measurement deteriorates.
In addition, the current ASTM cloud point method requires a considerable amount of an operator's time to make one determination. An operator's subjective judgement is also required to determine the onset of haziness in the sample.
Since the cloud point method has been established, numerous inventors have come forward with ideas to automate the measurement. Most of the ideas were centred around improvements related to the automatic detection of cloud formation and automatic charging and discharging of a sample cell. These systems tend to be expensive and have various drawbacks.
Thus, many prior art systems require complex and expensive cooling systems either because a large sample of liquid is required or because a fairly large chamber is cooled. Use of a large sample also gives rise to possible inaccuracies caused by lack of temperature uniformity. Such systems are described in:
U.S. Pat. No. 3,077,764 which issued Feb. 19, 1963 to Kapff;
U.S. Pat. No. 3,248,928 which issued May 3, 1966 to Conklin et al;
U.S. Pat. No. 3,527,082 which issued Sept. 8, 1970 to Pruvot et al;
U.S. Pat. No. 3,580,047 which issued May 25, 1971 to Simpson;
U.S. Pat. No. 3,643,492 which issued Feb. 22, 1972 to Simpson;
U.S. Pat. No. 3,447,358 which issued June 3, 1969 to Crespin et al; and
U.K. Patent No. 1,438,754 published June 9, 1976.
Other prior art systems have generally enclosed cells or containers through which the liquid sample is caused to flow. In addition to being relatively complicated, such arrangements may make cleaning of the cell or vessel difficult. Such arrangements are shown in:
U.S. Pat. No. 3,187,557 which issued June 8, 1965 to Holbourne;
U.S. Pat. No. 3,457,772 which issued July 29, 1969 to Chassagne et al;
U.S. Pat. No. 3,545,254 which issued Feb. 13, 1968 to Chassagne et al;
U.S. Pat. No. 4,519,717 which issued May 28, 1985 to Jones et al.
U.S. Pat. No. 3,807,865 (issued Apr. 30, 1974 to Gordon et al.) shows an arrangement in which a small sample of liquid is placed in a glass tube which has previously been sealed at one end, and which is then "drawn off and sealed as close to the upper meniscus of the sample as convenient". The tube is placed in a flowing heat transfer fluid to effect cooling; temperature is measured by a thermometer close to the tube. The presence of a solid phase is detected by monitoring for light scattered when a light beam is passed axially into the tube.
In most of the proposals described in previous patents, the cooling rate of the oil sample was either poorly defined or uncontrolled. For example, U.S. Pat. No. 3,187,557 suggests a quick cooling as it mentions a 60 times decrease in analysis time as compared with the ASTM method. In U.S. Pat. No. 4,519,717, a variable times shorter than the ASTM method. Such high cooling rates have been shown to result in inaccuracy.
It has also been suggested in U.S. Pat. No. 4,083,224 to Gayst (issued Apr. 11, 1978) that apparatus designed for dewpoint measurement might be used for measuring the freezing point of a liquid. This suggestion comes in a short last paragraph of Gayst, and no details are given. Gayst describes a dewpoint monitor in which light reflected off a mirror is monitored by a light detector and a reduction in such reflected light, due to scattering by droplets, is measured. Adopting such apparatus for use in determining freezing point would encounter some problems not addressed by Gayst, e.g.
(1) The reflected light would also be refracted when a liquid is present in the well, altering the light received by the light detector depending on depth of liquid and the refractive index, which will change as the temperature is lowered.
(2) The change in reflected light caused by the cloudiness typical of cloud point measurements would be so small as to be probably undetectable with Gayst's apparatus, especially since his reflected light detector can only "see" a central part of the mirror surface and would not detect crystals near to the side.
(3) Gayst provides no enclosure which would exclude ambient light, another reason why his device would likely be unable to detect small crystals.
(4) Gayst does not have any provision for dealing with a liquid or solid substance requiring a protective atmosphere.